Что за ошибка: Object reference not set to an instance of an object?

using UnityEngine;
using System.Collections;
 
public class bullet : MonoBehaviour {
 
    public Transform bullets;
    public Transform spawns;
    private float seeDistance = 20f;
    public float attackDistance = 20f;
    public float speed = 6;
    private Transform targets;
 
    void Shoot()
    {
        Transform fire = (Transform)Instantiate(bullets, spawns.position, spawns.rotation);
        fire.LookAt(targets);
        fire.transform.Translate(new Vector3(0, 0, speed * Time.deltaTime));
    }
 
    // Use this for initialization
    void Start () {
        targets = GameObject.FindWithTag("Enemy").transform;
    }
    
    // Update is called once per frame
    void Update () {
        if (Vector3.Distance(transform.position, targets.transform.position) < seeDistance)
        {
            if (Vector3.Distance(transform.position, targets.transform.position) > attackDistance)
            {
                transform.LookAt(targets.transform);
                Shoot();
            }
        }
        else {
 
        }
 
    }
}

this.transform.Translate( this.transform.forward *speed * Time.deltaTime);

fire.GetComponent<Rigidbody>().velocity = fire.transform.forward * speed;

void Shoot()
    {
        Transform fire = (Transform)Instantiate(bullets, spawns.position, spawns.rotation);
        fire.LookAt(targets);
        fire.GetComponent<Rigidbody>().velocity = fire.transform.forward * speed;
    }